Air springs are mounted between the axle and the chassis as supporting parts in a vehicle. The distance between the chassis and the axle should not change even for different loading conditions. For this reason, it is necessary to measure the air spring height and to readjust the system by pumping and venting air when there are deviations.
Methods for ultrasonic distance measurements are utilized in various areas. Air ultrasonic distance measurements are used, for example, for fill level measurements of vessels, to measure rooms in a building, to measure distance when parking a motor vehicle and to measure distance in autofocusing photo apparatus, et cetera.
Various ultrasonic distance measurement devices have been suggested for determining distances within air springs.
An ultrasonic sensor within an air spring is described for use in a vehicle in German patent publication 3,620,957. In U.S. Pat. No. 4,798,369, an air spring control system is shown which is based on ultrasonic sensors in the form of transducers for measuring spring elevation. The advantages of using ultrasound within an air spring is that no turbulence of the sound waves is possible because of the air stream and that a very good range of the ultrasonic wave is possible even at higher frequencies in the overpressure present in the air spring.
An ultrasonic sensor, which is built into the air spring, for measuring elevation can be disturbed when high frequency sonic oscillations occur when filling and/or venting the air spring. The sonic oscillations are caused because, when the valves are opened, a pressure wave runs into the air spring and flow noises occur at the connecting pieces and these flow noises have a frequency which reaches up to and into the ultrasonic range. A possibility for reducing these disturbances is to reduce the flow velocity, for example, by reducing the cross section of the air line. However, this possibility presents the disadvantage that the lifting and lowering operations take place only very slowly.